Circuit arrangement for producing a sawtooth current



March 31, 1970 J. J. REICHGELT ET AL 3,5

' CIRCUIT ARRANGEMENT FOR PRODUCING A SAWTOOTH CURRENT Filed Sept. 15.1966 6 Sheets-Sheet l INVENTOR JOHANNES LREICHGELT 5 WILHELMUSTH.H.HETTERSCHEID BY March 31, 1970 J. RElCHGELT ET AL 3,504,224

CIRCUIT ARRANGEMENT FOR PRODUCING A SAWTOOTH CURRENT Filed Sept. 15,1966 e Sheets-Sheet 2 INVENTORS JOHANNES J.REICHGELT WILHELMUSTH.H.HETTERSCHE!D 4004. 6.12%.; AGENT March 31, 1970 RE|HGELT ET AL3,504,224

CIRCUIT ARRANGEMENT FOR PRODUCING A SAWTOQTH CURRENT Filed Sept. 15,1966 6 Sheets-Sheet 3 l VBEZT H/ INVENTORS JDHANNES J.REI CH GELTWILHELMUS TH.H.HETTERSCHEID BY AGENT CIRCUIT ARRANGEMENT FOR PRODUCING ASAWTOOTH CURRENT Filed Sept. 15, 1966 March 1970 J. J. REICHGELT ETAL 6SheetsSheet &

INVENTORJ JOHANNS .LREICH can gXIILHEL MUS TH.H.HETTERSCHEID AGENT March31, 1970 J RECHGELT ETAL 3,504,224

CIRCUIT ARRANGEMENT FOR PRODUCING A SAWTOOTH CURRENT Filed Sept. 15,1966 6 Sheets-Sheet 5 INVENTORJ JOHANNES J-REICHGELT gILHELMUS TH.H.HETTERSC HEID AGENT March 31, 1970 J E T ET AL 3,504,224

CIRCUIT ARRANGEMENT FOR PRODUCING A SAW'I'OOIH CURRENT Filed Sept. 15,I966 6 Sheets-Sheet 6 INVENTOR5 JOHANNES J. RE! cu s ELT WILHELMUS TH.H.HETTERSCHEID w L Y AGENT United States Patent US. Cl. 315-27 12 ClaimsABSTRACT OF THE DISCLOSURE A deflection circuit that includes aunidirectional transistor having its collector connected to thedeflection coil. Inductive coupling means are provided between the baseand emitter for applying a switching signal thereto. The switchingsignal cuts off the transistor for a period that is longer than theflyback period whereby the deflection coil reverses polarity to forwardbias the base-collector junction of the transistor which allows areverse current to flow in the deflection coil via said base-collectorjunction to initiate the forward stroke of the deflection current. Theamplitude of the transistor supply voltage is at least ten times thepeak-to-peak amplitude of the switching signal.

The present invention relates to a deflection circuit for producing asawtooth current in the line-deflection v coils of a display tube. Thedeflection circuit is being provided with a transistor having an outputcircuit that includes the deflection coils. By means of an inductivecoupling between the base and the emitter electrode, a pulsatoryswitching signal is applied to the transistor which cuts off andreleases the transistor periodically.

Such a circuit arrangement is known inter alia from the US. Patent3,143,686. However, the transistor used in this circuit arrangement isof the symmetrical type (bi-directional transistor, of column 3, line 53of the aforesaid US. patent). A circuit arrangement having asymmetricaltransistor has the disadvantage, however, that the duration of theswitching pulses cutting off the transistor must be exactly equal to theflyback time of the sawtooth current since it is necessary to drive thetransistor immediately back into saturation at the beginning of thestroke time.

However, to drive the transistor so that its cut-off time is exactlyequal to the flyback time is much too critical for practical purposes,since the cut-off time is determined by the switching signal and dependsupon the adjustment of the oscillator supplying the switching signal,whereas the flyback time depends upon the value L of the overa linductance in the output circuit of the transistor, and upon the value Cof the stray capacitances and additional capacitances, if any, in thisoutput circuit. It is diflicult to choose for each case, especially inmass production, the conditions in the oscillator and in the said outputcircuit so that the aforementioned requirement of equality is fulfilled.Moreover, the equality originally adjusted accurately may besubsequently lost due to aging or other variation in the circuitparameters during the use of the circuit arrangement.

For all these reasons, a circuit arrangement having a symmetricaltransistor the line output transistor is not suitable for practical use.

Another known deflection circuit uses an asymmetrical transistor inwhich the damper diode is dispensed with by forward biasing thebase-collector junction of the transistor so that a reverse current canflow through the deflection coil via said base-collector junction at thestart of the stroke period. This circuit exhibits a discontinuity in thelinear portion of the sawtooth current at the point the current pathchanges from base-collector to emittercollector.

The circuit arrangement in accordance with the invention provides asolution which renders it suitable for practical use and it ischaracterized in that the transistor used is an essentially asymmetricalor unidirectional transistor. In addition, the duration of the pulses ofthe switching signal which cut-off the transistor is longer than theflyback time of the sawtooth current so that at the beginning of thestroke time, a current, reverse to that at the end of the stroke time,may flow through the deflection coils via the forward biasedbase-collector diode of the transistor, while the supply voltage of thetransistor exceeds by many times, for example, by ten times, thepeak-to-peak value of the switching signal applied between the base andthe emitter electrode. The inductive coupling includes an inductancedirectly connected between the base and the emitter of the transistor.

A few possible embodiments of circuit arrangements in accordance withthe invention will now be described more fully with reference to theaccompanying figures, in which:

FIG. 1 shows the basic circuit arrangement in accordance with theinvention,

FIGS. 2, 3 and 4 show equivalent circuit diagrams of the circuitarrangement shown in FIG. 1 for explaining its operation.

FIGS. 5a to 5g illustrate curves of currents and voltages that may occurin the circuit arrangement shown in FIG.

FIG. 6 shows a second embodiment of a circuit arrangement in accordancewith the invention in which the driver transistor is a symmetricaltransistor,

FIGS. 7a to 7g show curves of currents and voltages that may occur inthe circuit arrangement shown in FIG. 6,

FIG. 8 shows a third embodiment of the invention in which a capacitor isconnected between the base electrode and the collector electrode of theoutput transistor in order to suppress parasitic oscillations,

FIG. 9 shows a further developed circuit diagram of the circuitarrangement of FIG. 8,

FIG. 10 shows a further embodiment of the invention.

In FIG. 1, the transistor T is a driver transistor. The collectorcircuit includes a transformer 1. The primary winding 2 of thetransformer 1 connects the collector electrode of the transistor T tothe supply voltage source V,. The switching signal 3, originating froman oscillator, not shown, is applied between the base electrode and theemitter electrode of the transistor T The switching signal releases thetransistor T during the flyback time of the sawtooth current that willflow through the deflection coils L and during part of the stroke timeof the sawtooth current. The signal cuts oil? the transistor T duringthe remaining part of the stroke time. The secondary winding 4 of thetransformer 1 is connected between the base electrode B and the emitterelectrode E of a line output transistor T The collector electrode C ofthe output transistor T is connected to the parallel-combination of adeflection coil L and a capacitor C while the emitter electrode E isconnected to ground. Between the end of the said parallel-combinationremote from the collector electrode and ground there is connected asupply voltage source which supplies a supply voltage V for the lineoutput transistor T As will be described hereinafter, it is required forthe circuit arrangement in accordance with the invention that thevoltage V exceeds by many times the switching voltage across thebase-emitter diode of the transistor T For example, in the embodimentshown in FIG. 1, the voltage V is equal ice to 220 v., whereas thevoltage V amounts to only 7 v. The transformation ratio of thetransformer 1 is equal to 1:1. The voltage V and the transformationratio determine the peak-to-peak value of the ultimate switching signalwhich is operative between base and emitter electrodes of the transistorT via the secondary 4.

As stated in the preamble, the line ouput transistor T of FIG. 1 is of avery particular structure. In the first place this transistor must becapable of withstanding a very high voltage.

Secondly, this transistor must essentially be asymmetrical, whichimplies that the transistor T must be capable of withstanding a basecurrent which, at the beginning of the stroke time, is equal to thecurrent flowing through the collector circuit. Therefore, the baseregion of the transistor T must be calculated for conveying such a highcurrent. As will be explained hereinafter, finally a situation arises inwhich the base-collector diode D and the base-emitter diode D areunblocked simultaneously. As a matter of course, the transistor T mustbe suitable for use in this situation in which the diodes are bothunblocked and convey comparatively high currents. A possible method ofconstructing such a transistor has been described in a copending U.S.application, Ser. No. 587,456, filed Oct. 18, 1966.

In order to explain the foregoing and the operation of the circuitarrangement shown in FIG. 1, FIG. 2 shows the equivalent circuit diagramof the circuit arrangement of FIG. 1. In this figure, the transistor Tis represented as a switch T The transformer 1 is assumed to be an idealtransformer having an inductance value L and connected in parallel withthe base-emitter diode D In view of the fact that the transistor T is annpntype transistor, the anode of this diode is connected to the basepoint B while its cathode is connected to the emitter point E, whichpoints correspond with those indicated in FIG. 1.

The switch T is connected in series with the supply voltage source Vsupplying the voltage applied between the base electrode B and theemitter electrode E of the transistor T when the transistor T is in theconductive state, which corresponds with the closed state of the switchT in FIG. 2.

In FIG. 2, the diode D represents the base-collector diode of thetransistor T This diode is shunted by a source 5 which supplies theamplified emitter current 111 The equivalent circuit diagram of FIG. 2also includes the horizontal deflection coil L its bridging capacitanceC and the supply voltage source V As is apparent from FIG. 2, a currentsource ocI is not included. In fact, this source would have to beconnected in parallel with the base-emitter diode D but, as statedabove, the transistor T is essentially asymmetrical, which means thatthe current source ocI supplies a negligibly low current and maytherefore be omitted.

The switching signal 3 ensures that the transistor T operating as adriver transistor is alternately in the conductive and in thenon-conductive state. Consequently, a pulsatory signal of a value l+b)Vat the instant t and of a value (1+0) V at the instant I is producedacross the secondary 4. This can be explained as follows. If thetransistor T is conducting (switch T is closed), the primary 2 is at avoltage of V volt. Since the transformation ratio between the windings 2and 4 is approximately 1:1, the value of the voltage at the secondary 4is equal to that of the voltage at the primary 2. Consequently, in thetime interval from to t in which the transistor T is conducting, thevoltage at the secondary 4 is also equal to V volt. However, since theDC. component is lost due to the inductive coupling between the windings2 and 4, the switching signal V operative between the base electrode andthe emitter electrode of the transistor T will have an average value of0, which is represented by the line 6 in FIG. 5a. Since the area belowthe line 6 must consequently be equal to that above it, the voltageexceeds the line 6 by a value bV at the instant t and by a value cV atthe instant t As is apparent from FIG. 5a, the switching voltage V isnot constant during the time the transistor T conveys current, butslightly increases. This is due to the fact that during this period theemitter current I is not constant but increases linearly, as is apparentfrom FIG. 5 e.

Therefore, the voltage V has a negative value of V volt during the timeinterval from 0 to t However, at the instant t the voltage V jumps fromthe value V to a value +bV Hence, in FIG. 2, and also in the followingFIGURES 3 and 4, the voltage source connected with the time switch Tsupplies a direct voltage of V volt, i.e. the cut-off voltage operativebetween the base electrode and the emitter electrode of the transistor Tif the switch T is closed during the time interval from 0 to t At theinstant t the switch T is opened and at that instant the voltage at thebase electrode B is equal to +bV volt. The starting point lies at theinstant 0 and it is assumed that at this instant a current I flowsthrough the deflection coil L which current has the value indicated inFIG. 5b. At the instant O, the switch T is closed, which means that thetransistor T is cut off. The electromagnetic energy accumulated in thecoil L as, a result of the current flowing through this coil at theinstant 0, starts oscillating decrementally along a cosinusoidal curveso that the frequency and the flyback time are determined by the valuesof L and C This decremental oscillation occurs from the instant O to theinstant t i.e. the flyback time of the sawtooth current I The fact thatthis decremental oscillation actually dies out at the instant 1 can beexplained as follows. As a result of the cosinusoidal decrementaloscillation of the current I the voltage across the parallel circuitoscillates decrementally along a sinusoidal curve. Consequently, thevoltage V at the collector C of the transistor T follows a curve asshown in FIG. 5 during the time interval from 0 to Z This decrementaloscillation of the voltage continues until the collector electrode C ofthe transistor T becomes negative with respect to the base electrode Bthereof. This is the case at the instant t If the collector electrode Cis negative with respect to the base electrode B, the base-collectordiode D conveys a current 1 so that the situation arises which is shownin FIG. 3. If the switch T is closed, the diode D does not conveycurrent so that the current uI =0. Therefore, the current source 5 andthe base-emitter diode D are omitted in FIG. 3. However, if the diode Dis conducting, a constant voltage constituting the sum of the voltages Vand V is again produced across the deflection coil L The sum of thesevoltages therefore determines the slope of the sawtooth current whichflows from the instant 1 to the instant t Moreover, the decrementaloscillation of the circuit L C is finished at the instant t When thetransistor T has the aforementioned properties, it is achieved that thecollector current 1 flowing during the time interval from t to t isequal to the instantaneous base current 1 since the emitter current Iand the amplified emitter current I are both equal to 0. Therefore, thebase current 1 will have the same configuration as the collector currentI during the time interval from t to t which is clearly apparent fromFIGS. 50 and 5d. In FIG. 5c, the collector current I flowing during thetime interval from t to t is negative, since it is opposite to thecurrent aI which is the normal collector current when the transistor Tactually operates as a transistor. The current I =I flows back throughthe switch T During the flyback time from 0 to t when 1 is not yetpresent, the current through the switch T is determined by that is tosay by the value of the coil L and the applied voltage V During the timeinterval from t to t the current through the switch T is equal to I, I

This is apparent from FIG. 5g in which the collector current I is shown.Therefore, the current flowing back through the base circuit of thetransistor T reduces the current of the driver transistor T Theinterrupted line in FIG. 5g indicates the path of the current flowingthrough T if this effect did not occur.

This effect can be considered as an additional advantage of the circuitarrangement in accordance with the invention, since, as is apparent fromthe numbers stated in FIG. 50, the collector current I has a value of-0.6A at the instant 1 which value decreases to approximately 0.3A atthe instant 1 In this time interval the base current 1 is equal to thecollector current I Since the base current also has a comparatively highvalue owing to the low value of a during the remaining part of the timeinterval, i.e. from 1: to t.;, this means that the base current Iconstantly has a comparatively high value. 'This current must ultimatelybe supplied through the driver transistor T If the advantage of thedecrease in the collector current I during the time interval t to twould not be found, the full magnetization current for the transformer 1would have to be supplied by the transistor T in order to provide thepossibility of the desired base current I flowing during the timeinterval from t to 1' During the time interval from to t the collectorcurrent I would then have a configuration as represented by theinterrupted line of FIG. 5g. In consequence thereof, the averagecollector current Imgem would not have the value shown by thedot-and-dash line 7 in FIG. 5g but this value would be considerablyhigher. This would imply not only that the driving power supplied ismuch higher but also that the transistor T must be capable ofwithstanding a much greater dissipation. This is avoided by thefavourableadditional effect of the decreasing collector current I duringthe time interval t t 2.

At the instant t the switch T is opened and a situation arises as shownin the equivalent diagram of FIG. 4. As appears from FIG. a, when theswitch T is opened, the voltage at the base electrode jumps to a value-|-bV This implies that, when the switch T is opened, the voltage atpoint B jumps by (llb)V volt. As is apparent from FIG. 5 however, thevoltage at the point C will also jump by the same value, since thebase-collector diode D continues to conduct. For the voltage at theanode of diode D becomes positive so that the diode tends to remainconducting. The voltage across a conducting diode is substantially zero,however, so that a jump of the voltage at the point B results in a jumpof the voltage at the point C. Owing to the said voltage jump, thevoltage across the coil L is also varied. When the switch T was closedthe voltage across this coil was equal to V -l- V volt, and after theinstant t this voltage is equal to V bV volt. Since the voltage V isvery high, however (in the relevant embodiment 220 v.), and the voltageV is relatively low, in this case 7 v., the voltage (1+b) V volt, whichis the difference between the voltages across the coil L before andafter the instant t is negligibly low with respect to the voltage V Thebend occurring in the collector current 1 and hence in the current I asa result of this voltage jump is therefore also negligibly small.

This bend could therefore not be observed on an oscillograph.

Consequently, although the collector current 1 substantially does notvary when the switch T is opened, the base current I and the emittercurrent I of the transistor T and the collector current 1 of thetransistor T are varied indeed, since when the switch T is opened, thetransistor current I becomes 0.

Since the current I through the coil L cannot jump either and thiscurrent was equal to l -i-l before the instant t after the instant t thebase current 1 must be equal to I due to the disappearance of thecollector current I However, since the current I cannot jump, this jumpmust be etfected by the base current I As is apparent from FIG. 5d, thebase current I jumps by slightly more than 0.6A (from approximately 0.3Ato slightly more than 0.9A), which is just the value of the collectorcurrent I at the instant t Since the collector current I substantiallydoes not vary, this additional current must be passed by thebase-emitter diode D through which an emitter current I begins to flowwhich exhibits a jump equal to that of the current I Since, as statedabove, the diode D remains unblocked at the instant t and the collectorcurrent I is not varied, the existing situation remains actuallyunchanged and the diode D may act as a shortcircuit for the source 5 sothat the current el does not yet flow in spite of the fact that theemitter current I is no longer 0.

This situation is maintained approximately to the instant t; at whichtime the collector current 1 becomes 0. This means that at this instantthe electromagnetic energy has completely disappeared from thedeflection coil L Owing to the fact that by opening the switch T at theinstant t all the conditions for a normal operation of the transistor Thave already been fulfilled, the base current applied from the instant tkeeps the transistor T in the saturated state.

In the embodiment shown in FIG. 1, a has a low value so that also innormal operation the current amplification from the base to thecollector is small in the time interval from t to t With currents asshown in FIG. 5, u amounted to 1.5. Otherwise, the operation of the lineoutput transistor T in the time interval from t to 1 is similar to thatof :a conventional transistor. As a result, the current I and hence thecurrent through the deflection coil L will increase to a given valuewhich, as stated above, is determined by the voltage V bV,. At theinstant t the switch T is closed again and the cycle described before isrepeated.

Although in the foregoing it is assumed that the transistor T is annpn-type transistor and that the transistor T is a pup-type transistor,these transistors may be of any other type. Only the polarity of theapplied supply voltages V and V and the direction of winding of thetransformer 1 must be taken into account.

Alternatively, the collector C of the transistor T may be groundedinstead of the emitter E. In this case, it is desirable also to exchangethe parallel-combination of L and C with the supply voltage source V sothat in fact a grounded collector arrangement is obtained. The operationis analogous, however, to that of the arrangement of FIG. 1.

It is not always necessary to establish the coupling with the baseelectrode of the transistor T via a transformer. One coil could besufficient which is connected in parallel between the base electrode andthe emitter electrode of the transistor T In this case, the switchingtransistor T may be arranged between the supply voltage source V and thecoil.

A second embodiment of a circuit arrangement in accordance with theinvention is shown in FIG. 6. This embodiment differs from that of FIG.1 in that instead of an asymmetrical driver transistor T a symmetricaltransistor of the npn-type is used. As a result, the switching signalmust also be of opposite polarity and therefore has a configuration asrepresented by the signal 3' in FIG. 6. Furthermore, the drivertransistor T is now energized by means of an RC circuit consisting ofthe resistor 8 and a large capacitor 9. Moreover, the transformer 1 ischosen to be considerably larger than the corresponding transformer 1 ofFIG. 1. Consequently, the magnetization current required to maintain thedesired base'current 1 during the time interval from t to 1 may beconsiderably lower than in the corresponding case of FIG. 1, because thecurrent s Ix Lxt as shown in FIG. 3, flows through the coil L and willbe considerably lower in the circuit arrangement of FIG. 6 than in thatof FIG. 1 during the interval from to t (cf. FIGS. g and 7g).

Owing to the fact that during the time interval from 0 to I the currentI has assumed a much lower value due to the increase of L resulting fromthe increase of the transformer 1, whereas the collector current 1 whichbegins to flow at the instant t has not varied at all, the decrease ofthe collector current 1 will be such that this collector current changesits sign, which is clearly apparent from FIG. 7g. For this reason, thetransistor T of FIG. 6 is a symmetrical transistor. Of course, startcould in principle be made again from the fact that the inversion of thecollector current 1 would result in that the base-collector diode of thetransistor T would be unblocked. This would mean, however, that anadditional current would be derived from the oscillator supplying theswitching signal 3. This would have an unfavourable reaction on theoscillator so that its frequency could vary and consequently thesynchronization of the horizontal deflection could be lost. However,when a symmetrical transistor is used and when it is ensured that thetransistor T is constantly kept in the saturated state by the basecurrent 1 this risk can be avoided. This also influences the release ofthe transistor T for in the time interval from I to t the driver currentis negative. This means that the collector electrode of the transistor Tacts as an emitter. Consequently, the base-collector diode of thetransistor T in fact would have to be cut off by the switching-oifvoltage, which is virtually impossible due to the current superimposedfrom the collector circuit. Preferably the instant of switching off thetransistor T is therefore shifted from to 1 This means that the timeinterval from 0 to t as shown in FIG. 7, which holds for the circuitarrangement of FIG 6, is lengthened with respect to that shown in FIG. 5which holds for the circuit arrangement of FIG. 1. However, it should betaken into consideration that the instant r must absolutely precede theinstant i since from the instant t the transistor T must be able tooperate again as a normal transistor, which is possible only in case thetransistor T is cut off. As is apparent, however, from FIG. 7, there isstill a wide choice since the time interval from t to t constitutes afairly great fraction of the cycle T.

As appears from FIGS. 70 and 7d, the a for the transistor T of FIG. 6 ischosen considerably higher than in the embodiment shown in FIG. 1. Inthe case of FIG. 6, the a is chosen to be equal to 6. It will beappreciated, however, that both in the case of FIG. 1 and in that ofFIG. 6 other values may also be chosen for a. When a symmetrical drivertransistor T is used, the a of the transistor T may consequently assumeconsiderably higher values. Thus, a value of 0::10 or even higher isstill possible.

In the foregoing, it has always been assumed that V =220 v., while(1+b)V %7 v. This results in a ratio of holds for transistorT However,it will be appreciated that said ratio cannot be reduced withoutlimitation, since otherwise the bend in the sawtooth current I occurringat the instant t becomes excessively great. Moreover, at too low a valueof V the collector current 1 would have to assume too high a value,since the product of current and voltage determines the power requiredto obtain in a given coil L the desired deflection of the electron beam.Thus, if V is chosen lower, 1 must be chosen higher. In the timeinterval from t to t it holds that I =I so that a higher 1 results in ahigher 1 which might cause the base current to assume an excessivelyhigh value. It may therefore be said that a ratio between V and V of10:1 substantially is the minimum ratio at which the circuit arrangementcan operate satisfactorily.

As already stated above, in the circuit arrangement of FIG. 6, thepeak-to-peak value of the switching signal 3' must have a value suchthat the driver transistor T remains in the saturated state during thetime interval from t to t so that it is ensured that the switchingsignal 3' switches the transistor T and that the voltages applied to thecollector electrode cannot cause this transistor to be cut offprematurely.

It is further apparent from FIG. 7g that the average collector currentof the transistor T is very low due to the fact that this currentbecomes negative during the time interval from 1 to 13 This averagecurrent could even be reduced to 0 if it holds that O +O =O O O and Odenoting the areas shown in FIG. 7. However, in practice 0 -1-0 ispreferably chosen to be greater than 0 so that a positive current flowsthrough the resistor 8. However, this current may be kept very low bychoosing 0 to be only slightly different from O +O In this case, thedissipation in the resistor 8 is comparatively low in spite of the factthat a high resistance value is required for this resistor. The correctvalue of the voltage V produced across the capacitor 9 depends upon theratio between the areas 0 O and 0 which, with a given design of theoutput transistor T in turn depends upon the inductance value of thecoil L and upon the value of the resistor 8 at a given supply voltage VThe circuit arrangement shown in FIG. 6 therefore affords the advantage,in comparison with that of FIG. 1, that a separate low supply voltage Vneed not be available. Thus, it becomes attractive to derive from thiscircuit arrangement the supply voltages for the remaining parts of atransistorized television receiver. For in this case, a supplytransformer is economized. This may be achieved, for example, bycoupling the deflection coils L with a line output transformer and byproviding the transformer with a tapping from which pulses of lowamplitude can be derived. After being rectified and smoothed, thesepulses can produce the supply voltage for the remaining transistors inthe receiver. It must be taken into account that in this case theoscillator supplying the switching signal 3 is also supplied with thissup ply voltage. However, this supply voltage is not present before thetransistor T and T are driven. Therefore, in order to render the circuitarrangement self-starting, it must be ensured that this drive is broughtabout automatically when the supply voltage V is switched on. This maybe achieved, for example, by providing a feedback capacitor between thebase and collector electrode of transistor T so that this transistorbecomes self-oscillating when the supply voltage V is switched on.However, if the value of the feedback capacitor is correctly chosen,then when the switching signal 3' becomes avail able it has a greaterinfluence than the feedback action so that the transistor T againbecomes a normally driven driver transistor.

It follows from FIGS. 5b and 7b that the current flow ing through thecoil L before the instant t flows back through the capacitor C After theinstant 1 this capacitor current must be taken over by the collectorcircuit of transistor T This means that the collector current thereofmust jump from zero to the value that the capacitor current has at theinstant t This is possible owing to the fact that, as stated above, thebase collector diode D is released at the instant t As shown in FIG. 8,however, the connections with the coil L include stray inductances L If,due to the inductance L the current should be before the instant t whileafter this instant a current would abruptly begin to flow through thisin ductance, the inductances L are abruptly excited, which together withstray capacitances in the circuit gives rise to stray oscillations atthe beginning of the stroke time. This is the case if the capacitor C isconnected in parallel with the coil L In order to prevent the occurrenceof these stray oscillations, a further improvement of the circuitarrangement in accordance with the invention consists in that thecapacitor C is connected between the base electrode B and the collectorelectrode C of the transistor T From the instant t the capacitor currentis then simply passed on to the diode D which opens at this instant.This does not involve a change of the current path, An abrupt excitationof the stray inductances L and hence the ap pearance of strayoscillations are avoided.

Finally, FIG. 9 shows in greater detail a circuit dia gram of thecircuit arrangement of FIG. 8. It appears from FIG. 9 that the supplyvoltage V is obtained by a direct rectification of the AC supplyvoltage. For this purpose, the supply voltage is applied through theswitch S to a rectifier D which rectifies the supply voltage and which,after smoothing of this voltage by the elements R C and C producesacross the capacitor C the DC supply voltage V The supply voltage V isapplied to the line output transformer between its windings 11 and 12.These windings are split up in order to ensure that the voltage pulses14 and 15 produced at the ends of the windings 11 and 12, respectively,have opposite polarities. It is favourable that the deflection coil L beconnected in series with the linearity control member L and the blockingcapacitor C between the ends of the windings 11 and 12, since in thiscase stray inductances in the windings 11 and 12 cannot give rise toundesired interferences. As a result of the opposite polarities of thepulses 14 and 15, the long conductors extending from the ends of thewindings 11 and 12 to the deflection coil L slipped around the neck ofthe display tube will not emit radiation.

The transformer 10 further includes a high-voltage winding 16 whichsteps up the pulses so that, after being rectified in the high-voltagediode D they supply the acceleration voltage for the final anode of thedisplay tube. The acceleration voltage is fed to this final anodethrough the conductor 17.

Finally, it should be noted that if the 04' of the output transistor T"should not be chosen excessively high and if nevertheless the drivertransistor T should not be supplied from a separate supply source, thecircuit arrangement shown in FIG. 10 may be used. This circuitarrangement is a further development of that shown in FIG. 6. Thecapacitor C is arranged in the manner shown in FIG. 8 and the deflectioncoil L is connected in series with capacitor C in the manner shown inFIG. 9. The circuit has an output transformer 10, an additional diode 19connected to the secondary 20 of the transformer 10, and a resistor 21.

If l /zot' l0(a'=l /2 example of FIG. 5, oz' =l0 example of FIG. 7), thevalue of the base current 1 in the time interval t to L; lies betweenthe values indicated in FIGS. 5d and 7d. A higher base current in thistime interval implies that in the time interval 09 t a greater amount ofelectromagnetic energy would have to be introduced into the inductor Lof the driver transformer 1. The driver current I and consequently alsoits average value must therefore increase. The value of the current 1will also lie between the values indicated in. FIGS. 5g and 7g.

A higher average driver current lclgem results in a higher dissipationeven if the resistor 8 is reduced. (V =I .R dissipation E =I RTherefore, if a low-voltage transistor were used for the drivertransistor T the losses in resistor 8 would become excessively high.

The solution of this problem is illustrated in FIG. 10. Resistor 21 hasa double function. When the circuit arrangement starts after switchingon the supply voltage V the resistor 21 acts together with resistor 8 asa voltage divider the required supply voltage is supplied from thejunction of resistors =8 and 21 to the transistor T Resistors 8 and 21may both be high since the starting current is fairly low. This currentneed be only so high that the pulses V (cf. FIGS. 5 and 7 producedduring the fly-back time O t and induced in the secondary 20 have anamplitude exceeding the voltage at the junction of the resistors '8 and21. The diode 19 then becomes conducting at the peaks of these pulsesand consequently rectifies these pulses. The rectified voltage issmoothed by the network 9-21 so that an additional amount of energy issupplied to the supply circuit for the driver transistor T Thus, thiscircuit arrangement is automatically matched to the a of the outputtransistor T Therefore, if or. originally had a high value, Iclgem islow. The voltage drop across resistor 8 is -low and the diode 19 remainsblocked during the circuit operation. "If a decreases due to ageing,lclgem and consequently the voltage drop across resistor 8 increases sothat the diode 19 becomes operative. A satisfactory operation of thecircuit arrangement without unnecessary dissipation in resistor 8 isthus ensured in all circumstances.

Moreover, wider tolerances of a of transistor T are now permissible. Ifoz is high, the diode 19 does not become operative, whereas if u is low,the diode becomes operative.

What is claimed is:

1. A deflection circuit for producing a sawtooth current having a givenflyback period in the line deflection coils of a display tubecomprising, a unidirectional transistor having an output circuit towhich the deflection coils are connected, inductive coupling meansincluding an inductor directly connected between the base electrode andthe emitter electrode of said transistor, means for applying to saidinductive coupling means a pulsatory switching signal having apeak-to-peak amplitude which periodically switches the transistorbetween a cut-off condition and a saturation condition, the duration ofthe portion of the switching signal pulse which cuts off the transistorbeing longer than the flyback period of the sawtooth current so that atthe beginning of the forward stroke a voltage is developed across saiddeflection coils of an amplitude and polarity to cause a reverse currentto flow through the deflection coils via the base-collector diodejunction of the transistor, and a supply voltage coupled to thetransistor that exceeds by at least 10 times the peak-to-peak value ofsaid switching signal applied between the transistor base and emitterelectrodes.

2. A circuit as claimed in claim 1 wherein the inductive coupling meanscomprises a transformer having a primary windin and a secondary winding,means directly connecting the two ends of the secondary winding to thebase electrode and the emitter electrode of the transistor, a drivertransistor, a second source of supply voltage, means connecting theprimary winding in series with the emittercollector path of said drivertransistor to said second supply voltage, the amplitude of said secondsupply voltage being at least 10 times lower than that of the supplyvoltage of said unidirectional transistor.

3. A circuit arrangement as claimed in claim 2 wherein thetransformation ratio between the primary and the secondary windings ofthe transformer is 1:1.

4. A circuit arrangement as claimed in claim 2 wherein the inductancevalue of said inductive coupling means is chosen to be so high that thecurrent flowing through it during the flyback period of the sawtoothcurrent is lower than the inverse collector current that flows in saidunidirectional transistor immediately at the beginning of the strokeperiod via its base-collector diode junction, whereby the direction ofthe emitter-collector current .of the driver transistor reverses at thebeginning of the forward stroke.

5. A circuit arrangement as claimed in claim 4 wherein the drivertransistor is a symmetrical transistor.

6. A circuit arrangement as claimed in claim 5 wherein said secondvoltage supply source for the driver transistor comprises, a capacitor,a resistor, and means connecting the capacitor to the voltage supplysource of the unidirectional transistor by means of said resistor.

7. A circuit arrangement as claimed in claim 6 wherein the inductancevalue of the inductive coupling means is chosen so that said drivertransistor draws an average current differing from zero from the supplyvoltage source of the unidirectional transistor.

8. A circuit arrangement as claimed in claim 1 further comprising acapacitor that resonates with the deflection coils to determine theflyback period of said sawtooth current, and means connecting saidcapacitor between the base electrode and the collector electrode of theunidirectional transistor.

9. A deflection circuit for producing a sawtooth current having a givenflyback period in a deflection coil comprising, a unidirectionalsemiconductor device having emitter, base and collector electrodes,means for coupling said deflection coil to the output circuit of thesemiconductor device, inductive coupling means comprising an inductordirectly connected between the base and emitter of said semiconductordevice, means for applying to said inductive coupling means a controlvoltage wave having a peak-to-peak amplitude and polarity toperiodically cut off current flow in said semiconductor device,

the duration of the cut-off period of said voltage wave being longerthan said given flyback period so that a voltage reversal occurs acrosssaid deflection coil of a magnitude and polarity to forward bias thebase-collector junction of said semiconductor device during a portion ofthe period when said voltage wave cuts off the semiconductor device, anda source of direct voltage coupled to said collector electrode of amagnitude at least ten times the peak-to-peak voltage of said controlvoltage.

10. A circuit as described in claim '9 further comprising meansconnecting said deflection coil and said direct voltage source in seriesacross the emitter-collector path of said semiconductor device.

11. A circuit as described in claim 9 wherein said semiconductor deviceis connected in the common emitter configuration, said circuit furthercomprising a capacitor connected between the base and collector of saidsemiconductor device so as to resonate with the deflection coil duringthe flyback period of the sawtooth current.

12. A circuit as described in claim 11 further comprising a drivetransistor having an output electrode coupled to said inductive couplingmeans and to a source of direct supply voltage, a deflection transformerhaving a primary winding coupled to the collector electrode of saidsemiconductor device and a secondary winding, a diode connected betweensaid secondary winding and said source of direct supply voltage forcoupling a portion of the flyback pulses to said supply voltage source.

References Cited UNITED STATES PATENTS 8/1965 Schneider. 1/ 1967Goodrich.

